The present invention relates to a semiconductor device in which one or more of power semiconductor elements is/are mounted on a lead frame and then this assembly is insertionally molded with a resin and, particularly, to a semiconductor device in which a portion of the lead frame serving as an outward guided terminal stands up generally perpendicularly from the lead frame.
In semiconductor devices known as power semiconductor devices operating with relatively large electric power, it is an important problem as to how the heat generated by the semiconductor elements are efficiently conducted and dissipated to the outside under a sufficient withstand voltage characteristic.
Therefore, hitherto, the so-called a semiconductor device composed of a lead frame has been known, in which device the lead frame is used as a circuit conductor and one or more of power semiconductor elements is/are mounted on one surface of the lead frame and then a base substrate made from a metal having excellent thermal conductivity is joined to the other surface of the lead frame via a resin insulating layer, which is designed for realizing the compatibility between high dielectric withstanding voltage and low thermal resistance characteristics.
FIG. 8 shows an example of this semiconductor device composed of the lead frame, which is disclosed in JP-A-11-42663. As shown therein, a lead frame 13 is used, which frame is formed substantially integrally from a conductor plate of copper, etc. having a uniform thickness. This lead frame 13 is adhered to one surface (a top side surface) of a metal base substrate 15 via an insulating layer 18, whereby the lead frame can play a role as a pattern of the conductor on a usual circuit board.
A predetermined end portion of the lead frame 13 is bent so that it stands up generally perpendicularly from the bonding surface of the base substrate 15 and thus an outward guided terminal portion 17 is formed. The outward guided terminal portion 17 and one or more of power semiconductor elements 11 are electrically connected with each other to the lead frame 13 by mounting the power semiconductor elements 11 on the lead frame 13, and necessary portions are connected with each other by an aluminum bonding wire 16, whereby necessary circuits such as the main circuit, etc. of a power converter are formed.
The reason why the lead frame 13 is bent perpendicularly to the surface of the base substrate 15 is to keep an insulation distance from the base substrate 15.
At the same time, for the same reason, the bending position of the lead frame 13 comes inward from an end surface of the base substrate 15 toward the center thereof (right direction).
The lead frame 13 is adhered to the one surface (a top surface) of the base substrate 15 via the thin insulating layer 18 and, therefore, the heat generated by the power semiconductor elements 11 are conducted to the base substrate 15 via the insulating layer 18, thereby ensuring the dissipation of heat. Therefore, this base substrate 15 is fabricated from a metal plate composing a metal such as aluminum and copper, which are excellent in thermal conductivity.
The insulating layer 18 serves to adhere the lead frame 13 to the base substrate 15 and serves the function of physically spacing both apart and electrically insulating therebetween with each other. For this reason, the insulating layer 18 is made from a thermosetting resin such as an epoxy resin having a glassy-transition temperature of not less than 100xc2x0 C. and this material is used in the form of a resin sheet.
A molded outer package (not shown) is adhered to the base substrate 15 by an adhesive or the like, in which package a resin such as an epoxy resin is filled. Thus, the remaining portion on the lead frame 13 except for the terminal portion 17 extending perpendicularly from the bonding surface of the lead frame 13 to the surface of the insulating layer 18, and one or more of power semiconductor elements 11 mounted thereon by bonding with a solder layer 26 are encapsulated, whereby a semiconductor device is completed.
In the above prior art, no consideration is given to the fact the terminal portion of the lead frame stands up from the bonding surface of the lead frame to be fixedly adhered to the surface of the insulating layer and, therefore, it is a problem that manufacturing process becomes complicated.
Normally, according to the prior art, if a semiconductor device in which, as shown in FIG. 8, the terminal portion 17 stands up generally perpendicularly from the bonding surface of the lead frame to be fixedly adhered to the surface of the base substrate, it is necessary to form the lead frame in such a manner that the terminal portion is bent beforehand and then to adhere this bent lead frame to the base substrate.
This is because a lead frame placed on the surface of the base substrate in an unbent state generally comes into contact with the insulating layer including the outward guided terminal portion and fixedly adheres thereto, with the result that it becomes difficult to bend the lead frame later.
A manufacturing process of semiconductor devices in accordance with the prior art will be described as shown in FIGS. 9(a) to 9(c).
First of all, as shown in FIG. 9(a), a lead frame 13 is prepared, which frame is bent beforehand so as to form an outward guided terminal portion 17. This lead frame 13 is placed on the surface of a base substrate 15 with an insulation resin sheet 18xe2x80x2 for insulative adhesion interposed therebetween, and is then heated under pressure to thereby obtain the state shown in FIG. 9(b). The resin sheet 18xe2x80x2 sets hard and thereafter forms an insulating layer 18. As a result, the lead frame 13 is fixedly adhered to the surface of the base substrate 15.
Next, similarly as shown in FIG. 9(b), solder printing treatment necessary for the formation of a solder layer 26 is exerted on the surface of the lead frame 13 for mounting one or more of the semiconductor elements 11 thereon. Subsequently, as shown in FIG. 9(c), the semiconductor elements 11 are superimposed on the solder layer 26 and the semiconductor elements 11 are joined to the lead frame 13 by heating under pressure. After that, a predetermined wiring is performed by means of a bonding wire 16.
In the prior art, however, there arose the following problems in these steps and thus the manufacturing process was obliged to become complicated as mentioned above.
The outward guided terminal portion 17 stands up from the lead frame 13 and, therefore, first of all, in the step of FIG. 9(a), it is necessary to prevent the interference (i.e., the collision) of the outward guided terminal portion 17 with a compression press and the like. This leads to an increase in equipment size and hence an increase in the amount of investment.
Next, in the step of FIG. 9(b), a mask and a squeegee for solder layer printing interfere with the outward guided terminal portion 17. Therefore, it is difficult to perform solder printing treatment for forming the solder layer 26 on the lead frame 13 and techniques such as screen printing cannot be applied thereto.
Further, in the step of FIG. 9(c), the outward guided terminal portion 17 stands a great physical difficulties to the operating range of mounting equipment necessary for mounting parts on the lead frame 13 and it is difficult to solve this problem with a usual mounting equipment.
The object of the present invention is to provide a semiconductor device composed of a lead frame in which the possibility of a complicated manufacturing process is prevented and a reduction in manufacturing cost can be satisfactorily achieved.
The above object is achieved by providing a semiconductor device in which a lead frame fixedly adhered via an insulating layer to one surface of a base substrate serves as a circuit conductor and on which frame one or more of semiconductors are mounted so as to bend an end portion of said lead frame in a stand-up direction from the bonding surface of the lead frame to be fixedly adhered to the surface of said base substrate as an outward guided terminal portion, wherein, in a portion of the lead frame, there is formed a recessed portion whose thickness is reduced from the bonding surface of the lead frame to be fixedly adhered to the surface of the base substrate and the lead frame is bent at this recessed portion having a reduced thickness.
The stand-up position of the lead frame from the surface of the base substrate may be spaced apart by a predetermined specific distance from an end surface of the base substrate. A curved portion following a straight portion may also be formed in a stand-up portion of the lead frame from the surface of the base substrate.